The usual planetary-gear train with three coaxial components, i.e., a sun gear, a surrounding ring gear and a planet carrier having one or more pinions in mesh with both gears, is widely used for automatic gear shifting. In such a gear train one of the components acts as a driving element, another is a driven element and the remaining one serves as a control element providing a reaction force upon being immobilized, thereby reducing the originally existing two degrees of freedom of the gear train to a single degree. Coupling elements for operatively connecting the driving component with an input shaft and/or the driven component with an output shaft as well as for immobilizing the control component comprise clutches and brakes, usually of the hydraulic type. With suitable combinations of a pair of such three-component gear trains, together with the associated clutches and brakes, it is possible to establish three forward speed ratios and one reverse speed ratio as is common in conventional automotive transmissions. Selection of any of these speeds requires the simultaneous actuation of two coupling elements, generally a clutch and a brake except in the case of direct drive (transmission ratio 1:1) for which two clutches are concurrently operated.
In order to increase the number of available speed ratios beyond the four referred to above, more than two planetary-gear trains may be connected in cascade as described, for example, in commonly owned U.S. Pat. No. 3,820,418. These prior transmissions are satisfactory for their intended purpose but are only limitedly adaptable to different operating conditions calling for either an increase or a decrease in the number of available speed ratios.